Method of etching fine filamentary apertures in thin metal sheets



Jan. 21, 1969 J, J, F Z Y 3,423,261

METHOD OF E ING FI AMENTARY APERTURE THIN META HEETS Filed March 1965 7/A '.Z i-

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Fig 7 JOHN J. FRANTZE/V BY W 7 ATTNEYS United States Patent 3,423,261METHOD OF ETCHING FINE FILAMENTARY APERTURES IN THIN METAL SHEETS JohnJ. Frantzen, St. Paul, Minn., assignor to Buckbee- Mears Company, St.Paul, Minn a corporation of Minnesota Filed Mar. 8, 1965, Ser. No.437,969 US. Cl. 15611 Int. Cl. C23f 1/02 2 Claims ABSTRACT OF THEDISCLOSURE This invention relates generally to the production ofprecision articles by etching. More particularly the invention isdirected to a process for producing improved articles of metallic,sheet-like construction containing a plurality of precision-formedfilamentary apertures and in which the articles are of sufficientthickness relative to their other dimensions to be substantially rigidor inflexible so that they are mechanically stable.

Experience has taught that in the art of making precision articles byetching a metallic plate, as the thickness of the plate increases thequality of definition of the etch decreases. This is principally due tothe undercutting effect of the etchant which is a natural result of theetching time period being greater near the surface of the plate to whichthe etchant is applied as compared to the other more further removedsurface. Where physical stability or rigidity of the finished articlehas been an important factor requiring that the article be produced froma relatively thick sheet of metallic material, a bimetal process isoften used. A type of bimetal process for etching a metal plate isdescribed in the Golay Patent 2,829,460. As described more thoroughlytherein, this process involves coating a relatively thick plate of thebasic metal with a thin layer of a different metal which in turn iscoated with a non-metallicprotective material. In those areas whereapertures are to be etched, the protective coating is removed and anetchant is applied to the thin metallic layer to remove this materialfrom the unprotected areas. This etchant does not attack the thickerbase metal. Sharply defined, precise dimensions can be maintained inetching through this thin metal layer. As the next step, an etchantwhich will not attack the thin metal layer is applied to remove the basematerial underlying the exposed areas. Even though the dimensions of theaperture formed through the thicker layer may be indefinite, theeffective dimensions of the aperture are defined by the etched-outportion of the thin metal layer and therefore the degree of precisionwhich is required is maintained. There are variations in this bimetalprocess. For example, the thin metal layer pattern can be electroformedonto the thicker base material rather than etched out. In any event, inorder to overcome the effect of undercutting which limits the degree ofprecision and definition, the two metallic layers are of differentmaterials so-that they are not both attacked by the same etchant. Theprincipal disadvantage of the bimetal process is that the article isthen produced from two different metals which have differentcharacteristics which does create problems in some areas of use.Specifically, articles 3,423,261 Patented Jan. 21, 1969 "ice constructedby the bimetal process which have been used in environmental conditionsin which relatively large variations in temperature take place, havebeen damaged or otherwise rendered useless because of the differentcoefficients of expansion of the metals involved. It is the object ofthis invention to overcome the disadvantages of the bimetal processwhich have been encountered heretofore.

Articles constructed according to the teachings of the instant inventioncan be considered, in a general sense, to be made according to a bimetalprocess but yet the finished article has virtually all the physicalcharacteristics of an article which is constructed from a singlemetallic material. Briefly, in this improved process, the relativelythick strengthening base sheet of a first metallic material is coatedwith a very thin layer of a different metallic material. The patternwhich defines the apertures to be formed, is laid out over this coatingwith a thin layer of metallic material which is the same as that in thebase sheet. This may be done by electroforming or etching or the like.The base sheet can then be selectively etched from its underside inthose areas corresponding to where the apertures are desired. The verythin coating layer will prevent the etchant from attacking thepattern-defining, top, thin layer of the same metal. When the base sheethas been suitably etched, the very thin coating material can be quicklystripped away because of its thinness so that apertures passingcompletely through the article are formed. As in the bimetal process,the thin layer defines the precision dimensions for the aperture.Because the great bulk of the metallic material in the finished productis to such an overwhelming degree a single metallic material, thearticle has, for all practical purposes, all of the physicalcharacteristics of an article constructed solely from this singlematerial.

Features and advantages of this invention will become apparent duringthe course of the following detailed description with reference to theaccompanying drawings in which:

FIG. 1 is a fragmentary top view of a rectangularshaped metallic platearticle containing filamentary apertures made according to the teachingsof thi invention.

FIGS. 27 are sectional views taken along line 7-7 illustrating the samearticle as it appears during various stages of the process of thisinvention.

An example of the type of articles that can be constructed according tothe teachings of this invention is illustrated in fragmentary form inFIG. 1. This article, which is identified generally by the numeral 10,may be a flat, relatively rigid 3" x 3 square sheet with suitablelocating or mounting holes, not shown, at its edges. This article 10contains a plurality of filamentary apertures 11 which may all be ofequal size or may vary in size and pattern as desired, depending on theultimate use for the article. Articles of this nature may be used asstencils, as viewing masks, or a variety of other uses. In any event, itis contemplated that the intended use is such that the article must bestructurally stable, i.e., relatively rigid or inflexible, and theapertures must be formed to a relatively high degree of precision.

FIG. 7 is a view of the finished article taken along section 77 ofFIG. 1. FIGS. 2-6 illustrate the condition of the article during varioussteps of the inventive process. Although in the following detaileddescription of the invention certain materials will be specified for usein the construction of the article, it should be understood that avariety of materials can be utilized providing they are compatible withthe various processing steps included in the invention and as recited inthe appended claims.

The base supporting layer 12 may be a 3" x 3 sheet of nickel having athickness in the order of .005 inch.

This order of thickness as compared to the other dimensions of the basesheet affords the latter a substantially rigid or inflexibleconstruction as required for the intended use. The top surface of thebase sheet 12, after being suitably cleaned, is coated with a very thinlayer of gold 13 which may be flashed onto the nickel surface to athickness in the order of .0001 inch, as illustrated in FIG. 2. Goldplating to a thickness in this order of magnitude relative to thethickness of the base sheet 12 will in general adequately serve itsintended function, which will be apparent shortly, but will havenegligible effect on the structural characteristics of the finishedarticle. The upper exposed surface of the gold layer 13 is then coatedwith a suitable light sensitive enamel and the desired pattern offilamentary apertures is photoprinted thereon in the usual andwell-known manner. The photoprinting step results in the protectiveenamel 14 covering only those areas of the gold plating whichcorresponds to the areas defining the filamentary apertures which are tobe formed, as seen in FIG. 3. The goldplated nickel article containingthe partial enamel coating is then placed in suitable electroplatingbath and a thin layer of nickel 15 is deposited on the gold plating inthose areas not covered by the protective enamel 14, in the well-knownmanner. The nickel is preferably electroplated in this manner, and asillustrated in FIG. 4, to a thickness in the order of .0004 to .0005inch. Next, the bottom surface of the base sheet of nickel 12 is coveredwith a protective coating of light sensitive enamel and the negativeimage of the pattern which was photoprinted on the gold plating isphotoprinted on this bottom surface in the well known manner. As aresult of this photoprinting step, protective enamel 16 remains on thoseareas of the bottom surface which underlie those top surface areas whichare covered by the thin nickel layer 15 over the gold plating as seen inFIG. 5. There is an absence of protective enamel on the bottom surfacein those areas underlying the top surface areas which are coated withthe protective enamel 14 over the gold plating. As stated earlier, thosetop surface areas covered by the enamel 14 so that they are void ofthin-layer nickel 15, define the filamentary apertures which are to beformed. A suitable etchant is then applied in the well known manner tothe bottom surface of the base nickel 12 so that it attacks the nickelin those areas unprotected by the enamel 16. FIG. 6 illustrates thegeneral pattern of the openings etched through the relatively thick basesheet nickel 12. This tapered effect is due to the fact that the etchanthas a longer reaction time with the material closest to the surface atwhich it in applied than it has with the material at the furthersurface. The etching of the base sheet 12 is continued until the basesheet nickel which underlies the enamel coated areas 14 has beenremoved. It goes almost without saying that the etching should not beallowed to continue to an extent where it would cause a structuraldeficiency by over-etching the base sheet. The etchant which is appliedto the base sheet 12 does not attack the pattern-defining thin layer ofnickel 15 since it is shielded from the latter by the very thin goldplating 13.

After etching the areas of enamel 14 and 16 are removed, eitherchemically or by stripping, so that the article appears as illustratedin FIG. 6. Finally, the gold plating 13 is removed from those areasunderlying the aperture-defining areas of the upper thin layer ofnickel. Because the gold plating is so very thin, it is readily strippedaway or it can be etched away chemically. The end result is asillustrated in FIG. 7. The filamentary apertures, 11, shown in sectionalview are effectively defined by the thin layer of nickel 15 with thedesired precision even though etching done through the relatively thicksupporting base sheet 12 would not have provided the degree of precisionrequired. The minute amount of gold in the finished article, as comparedto the great bulk of nickel, with the gold limited to an extremely thinplating or flash coating, has negligible effect on the physical andstructural characteristics of the finished article. Thesecharacteristics are then determined for all practical purposes by thedominant nickel material.

Although in the foregoing description, the aperturedefining pattern isformed by the upper thin layer of nickel being electroformed ordeposited over the gold plating, this can be done in other ways. Forexample, instead of coating the gold plating 13 with enamel, a uniformthin layer of nickel can be placed over the gold plating by, forexample, electroforming to a thickness in the order of .0004.0005 inch.This thin layer of nickel can then be coated with enamel and a suitablepattern photoprinted thereon. In this event, the pattern would be thesame as that photoprinted on the bottom surface of the base sheet nickel12, such as illustrated in FIG. 5. The thin layer of nickel would thenbe etched and only those areas not protected by the enamel, which wouldcorrespond to areas 14 shown in FIG. 5, would be etched away leaving theother areas of nickel, 15, intact in the same manner as illustrated inFIG. 5. Electroforming the thin layer of aperture-defining pattern ofnickel has been found preferable, however, since it provides betterdefinition of the filamentary apertures.

Articles constructed according to this invention have been used inenvironmental conditions in which the temperature has varied over a widerange with no resulting impairment in the physical structure norreduction in the effective use thereof. This can be compared to thedifiiculties encountered where a different metal has been used in thebimetal process for forming the aperture-defining pattern resulting inloss of reliability and permanent damage.

I claim:

1. A method for producing a substantially inflexible metallic sheetarticle containing a pattern of a plurality of precision filamentaryapertures, comprising the steps of:

(a) coating a surface of a relatively thick substantially inflexiblebase sheet of nickel with a flash coating of gold in the order of thethickness of the base sheet of nickel;

(b) forming on the gold flash coating a thin layer of nickel arranged ina pattern defining a plurality of filamentary apertures, said thin layerbeing substantially thinner than the base sheet yet not less than fourtimes the thickness of the gold flash coating;

(0) chemically etching away areas of the base sheet of nickel whichunderlie the aperture areas in the thin layer of nickel with a chemicaletchant which does not react with the gold flash coating; and

((1) then removing the areas of the gold flash coating corresponding tothe aperture areas of the nickel.

2. The invention as described in claim 1 wherein the patterned thinlayer of nickel is formed on the gold flash coating by:

(a) first covering the gold flash coating with a lightsensitive enamel;

(b) then exposing the enamel to actinic light so that only the areas ofthe enamel corresponding to the desired aperture areas are hardened bythe light from the source;

(c) removing the unhardened areas of enamel; and

(d) electrodepositing nickel to the desired thickness on the gold flashcoating in those areas from which the enamel had been removed.

References Cited UNITED STATES PATENTS 2,469,689 5/1949 Gresham 204-242,649,361 8/1953 Springer et al 156-48 JACOB H. STEINBERG, PrimaryExaminer.

US. Cl. X.R.

